The invention relates generally to a wrench and fastener arrangement and more particularly to a wrench and fastener arrangement having a higher torque to failure in the loosening direction than in the tightening direction.
Fasteners come in a variety of types. For example, there are nuts, which are threaded onto a metal shaft and bolts which may threadingly receive a nut, or be threadingly received in a bore. Fasteners usually have a head that includes surfaces for loosening and surfaces for tightening. On many fasteners, the loosening surfaces are the same as the tightening surfaces (for example, a hex head nut or bolt). Other fasteners utilize surfaces for loosening that are different than the surfaces utilized for tightening. The loosening surfaces and tightening surfaces may be formed on the outer perimeter of the fastener head (hereinafter an xe2x80x9cexternal fastenerxe2x80x9d) or on a periphery formed inside of the outer perimeter (hereinafter, an xe2x80x9cinternal fastenerxe2x80x9d). External fasteners are designed to be used with open end wrenches or closed end wrenches, such as box wrenches or socket end wrenches. Internal fasteners (among which are so called xe2x80x9cAllenxe2x80x9d fasteners) are generally designed to be used with an internal-key wrench. Some wrenches (such as an open-end adjustable wrench) are designed to fit a variety of fastener sizes and configurations.
The periphery on which the loosening surfaces and tightening surfaces of a, fastener are formed is referred to herein as a fastening periphery. While many fasteners have the loosening surfaces and tightening surfaces formed on the same fastening periphery, it is also known to form loosening surfaces on one fastening periphery and tightening surfaces on a different fastening periphery. Such a design has the disadvantages of (1) being relatively expensive, and (2) having too large a fastener head to practically be used in certain applications.
Known wrenches and fasteners have primarily been designed symmetrically, transmitting torque equally in both the tightening and loosening directions. Typical socket wrench types of this kind are shown in FIGS. 1-3. FIG. 1 shows a 1xc2xdxe2x80x3 Hex socket, FIG. 2 shows a 1xe2x80x3 12 point socket, and FIG. 3 shows a 1xe2x80x3 12 point spline. However, the torque required for loosening a fastener that has been tightened is several times more than the torque required for tightening a fastener. This is because metal surfaces in contact with one another for an extended period of time tend to seize and resist separation. Another factor that causes the fastener to resist separation is the dissolution of the lubricant that may have been present at the time of tightening. Additionally, tightening of a fastener to near its ultimate strength will cause permanent deformation. This permanent deformation causes the pitch of the threads on the fastener to no longer precisely match the pitch of the nut or the tapped hole. The mismatch requires additional torque to force the threaded elements to conform sufficiently to allow rotation of the fastener. The application of the additional torque required to loosen a fastener can result in system failure (sometimes referred to herein simply as xe2x80x9cfailurexe2x80x9d) prior to the fastener being loosened.
There are numerous modes of fastening system failure. When failure occurs, the mode of failure depends on both the design and physical properties of the wrench and fastener, including their respective strengths, hardnesses and ductilities. A socket wrench may split because of the combined circumferential and radial forces, or its teeth (referred to herein as protuberances) may shear or bend because of the combination of radial and circumferential forces. Or the teeth may flow from excessive contact pressure. Further, the points (referred to herein as protuberances) of the fastener may shear, bend or flow. In addition, in the case of a hollow-head fastener, the fastener head may split due to a combination of radial and circumferential forces, or the wrench may fail in torsion. Existing fasteners and wrenches tend to have a single mode of failure for each particular wrench and fastener. There are numerous modes of failure for both wrenches and fasteners that must be considered so that if steps are taken to a strengthen against one failure mode it does not result in increasing the likelihood of another failure mode.
Fastening system failure is expensive because of increased labor and the cost of providing new wrenches and/or fasteners. For example, if a bolt fails the damaged bolt must usually be drilled and removed with special tools. Fastening system failure can also be dangerous because a user applying a great deal of force to a fastener can be harmed when the system fails and his hand or arm strike a hard or sharp object. As it will be understood, fastener system failure most often occurs when attempting to loosen a fastener because of the greater torque required.
In summary, it would be an advantage to provide a fastening system capable of generating sufficient torque to loosen a fastener, have a fastener and wrench design that can withstand the force of generating such torque without failing, and produce the system in the same dimensions as existing fastening systems.
The present invention solves these and other problems by providing an asymmetrical fastening system comprising a fastener and a wrench. The fastener has a single fastening periphery and the wrench has a single fastening periphery designed to engage the fastener periphery. A plurality of loosening surfaces and a plurality of tightening surfaces are formed on the fastening periphery and on the wrench fastening periphery. The system is asymmetrical because it is capable of generating more torque to loosen the fastener, and solve the hereto-forementioned problems, than tightening a fastener.
As used herein, the term radial refers to a vector extending outward in a straight line from the center of a fastener. In the preferred embodiment, the fastener has loosening surfaces formed at an angle closer to radial than the angle of the tightening surfaces. Therefore, the radial forces, i.e., the forces directed along a radial vector, which tend to damage the wrench or fastener, are less when a given torque is applied in the loosening direction than when the same torque is applied in the tightening direction. The circumferential force is the same in both cases. The circumferential forces transmit torque from the wrench to the fastener and are, therefore, the ones that tighten or loosen the fastener. The radial forces add to the stresses imposed on both the wrench and the fastener and work in conjunction with the circumferential forces to increase the total forces. The result is that the torque to failure of the system in the loosening direction is greater than the torque to failure in the tightening direction.
The reduction of radial forces can not be carried too far without causing failure from increased contact pressures and reduced shear areas. Thus, balance in design is necessary. But greater loosening strength can be achieved by designing the system to generate greater loosening torque without failure of the system, which requires the mechanical advantages of the system to be in favor of loosening. Thus, the best design is asymmetrical.
In accordance with the preferred embodiment, there is provided an asymmetrical fastener that can engage the asymmetrical wrench so that the fastener can be axially rotated to a loosened or tightened state. The wrench can be a socket type, a box end type wrench or an internal-key wrench type. The wrench preferably includes a plurality of protuberances and a plurality of recesses formed about a central axis in a single fastening periphery. Each protuberance includes a wrench loosening surface for loosening a fastener and a wrench tightening surface for tightening a fastener. The wrenching loosening is preferably formed at a much smaller angle, with respect to a radial vector extending from the wrench""s central axis, than the wrench tightening surface. The result is that the torque to failure of the system is greater in the loosening direction. Another advantage of the present invention is the overall system improvement while maintaining the same size fastener head. The load transmission increases proportionally to the number of protuberances or teeth. Therefore, the number of teeth can vary depending on the application or particular torque requirements. Additionally, more teeth allows the use of a smaller head diameter for a given fastener""s size because the difference between the major and minor diameters is less.
Also disclosed herein is a wrench protuberance having a greater area where shear forces act to reduce the likelihood that the protuberance will break or bend while in use.